1. Field of the Invention
This invention relates to ultra-fine gold particle-immobilized alkaline earth metal compounds, methods for the production thereof, and oxidizing catalysts, reducing catalysts, and inflammable gas sensors each having the ultra-fine gold particles immobilized with alkaline earth metal compounds as a substantially main component thereof.
2. Prior Art Statement
It has been known that ultra-fine gold particles having a particle diameter smaller than about 0.1 .mu.m exhibit specific physical and chemical properties different from those of the ordinary coarse gold grains ("Ultra-fine Particles" published by Agne Publishing Center in 1986).
Generally, ultra-fine particles possess large surface energies, coagulate easily and are therefore hard to handle. Particularly, the ultra-fine particles of gold exhibit a strong binding property between the individual particles and tend to agglomerate as compared with the ultra-fine particles of other noble metals such as Pt and Pd. Therefore, the development of the methods to deposit and immobilize ultra-fine gold particles on a carrier in a uniformly dispersed state has been desired.
Generally, the surface reactivities of the carriers vary with their kind of compound. Therefore, the methods suitable for the immobilization of the ultra-fine gold particles on the carrier also vary with the kind of carrier to be used. The only known way for effecting the immobilization of the ultra-fine gold particles, is a method which is utilized for only a few compounds such as, for example, oxides of manganese, iron, cobalt, nickel, and copper. These compounds have been studied with respect to a catalytic property and various other properties (Japanese Patent Public Disclosure SHO No. 60(1985)-238148). This method, however, is not totally practicable because the conditions for the immobilization of the ultra-fine gold particles are complicated and the amount of gold used is large. A method which comprises immersing a carrier in an aqueous solution containing a soluble gold compound and precipitating gold hydroxide on the carrier through the hydrolysis of urea or acetamide has been known in the art (U.S. Pat. No. 4,698,324). However, this method necessitates elaborate control of the conditions for the precipitation of gold and, at the same time, suffers from a poor gold utilization efficiency. The inventors made a study with a view to developing practical ultra-fine gold particle-immobilized compounds and a method for the production of the compounds. They consequently invented ultra-fine gold particle-immobilized compounds using various metal oxides as carriers and several methods for the production thereof. They applied for a patent of this invention in the United States (U.S. patent application Ser. No. 07/171,810, now U.S. Pat. No. 4,839,327).